An ink-jet cartridge structure for a conventional ink-jet printing apparatus, as disclosed in U.S. Pat. No. 5,409,134, provides a fluid pressure regulator in the ink reservoir for controlling the back pressure within an operating range that is suitable for preventing ink leakage while permitting the ejecting element to eject ink drops during printing. As shown in this prior patent, the fluid pressure regulation in the rigid ink reservoir can be controlled by a flexible bag that is mounted to a curved spring. The pressure within the rigid ink reservoir drops instantly as the ink is ejected from the rigid ink reservoir via the ejecting element. At this moment, the ambient pressure is larger than the pressure within the rigid ink reservoir. Thus, ambient air will flow into the flexible bag so that the ink can be ejected smoothly while printing. Moreover, the expansion of the bag causes the deflection of the adjacent springs and thus decreases the ink pressure in the ink reservoir so that ink leakage can be prevented during printing.